1. Field of the Invention
The present invention relates to the field of the control of burners of a fuel-fired system and, more specifically, to the control of the input rate of ON-OFF burners by varying the cycle time (hereinafter referred to as "pulse spacing").
2. Prior Art
It has been state of the art to vary a directly controlled variable by shifting the trailing edge of the ON-pulse to vary the duty factor correlating the pulse duration and the pulse separation, keeping the pulse spacing constant. If such a control method is adopted, both the pulse duration and the pulse separation are changed. However, if said control method is applied to control ON-OFF-controlled burners of a fuel-fired system, it is a disadvantage that such burners can only be operated at medium load, since burners will only emit oxides of nitrogen at a relatively low rate, and other combustion conditions will only be satisfactory, if said burners remain ON for a first minimum period, depending on burner design, forming part of the total firing cycle, and remain OFF for a second minimum period also depending on burner design. The useful (medium-load) range for the operation of such burners could be widened by increasing the time length of the total firing cycle, but such an increase would cause the response of the control system to be slower. The present invention has remedied said disadvantage.